A plasma potential is higher than its surrounding potential. This will be explained by using as an example a parallel plate type plasma processing apparatus 99 shown in FIG. 8. In a plasma processing space in a processing chamber 900, when a bias potential is negative (negative wafer potential), i.e., when a wafer potential Vwafer is lower than a wall potential Vwall (i.e., ground), a plasma potential Vplasma becomes higher than the wall potential Vwall. Meanwhile, when a bias potential is positive (positive wafer potential), i.e., when the wafer potential Vwafer is higher than the wall potential Vwall, the plasma potential Vplasma becomes higher than the wafer potential Vwafer.
A potential difference between the wall of the processing chamber 900 and a plasma (Vwall−Vplasma) greatly affects productivity of an etching process. In other words, if the potential difference (Vwall−Vplasma) is too large, a sputtering force of ions in the plasma on the wall surface is increased, and radicals in the plasma may not be deposited easily on the wall surface. Therefore, the wall of the processing chamber is eroded, and this causes production of particles, contamination in the chamber, erosion of components or the like.
On the other hand, if the potential difference (Vwall−Vplasma) is too small, a sputtering force of ions in the plasma on the wall surface is decreased, and radicals in the plasma are easily deposited on the wall surface. Thus, reaction products are deposited on the wall, which leads to formation of a film. For example, when a process using a CF-based gas has been performed in a previous process, a CF film (polymer) is formed on the wall surface of the processing chamber during the process. In that state, if a process using O2 gas is performed in the same processing chamber in a next process, a plasma is generated in a state where O2 and CF are mixed. Hence, components of the CF film adhered to the wall surface enter the plasma and react chemically with other substances, thereby inflicting adverse effects on a desired plasma process. This is a so-called memory effect problem. In addition to the memory effect problem, there is generated another problem in which the inside of the processing chamber needs to be cleaned frequently as the amount of the film adhered to the wall is increased. This leads to a decrease in the productivity and an increase in the manufacturing cost.
Recently, due to a growing user's demand to improve a throughput, processing time is shortened by increasing an etching rate or the like. In order to meet this demand, a high frequency power of a higher level needs to be supplied into the processing chamber. When the high frequency power of a higher level is outputted from the high frequency power supply, a sputtering force on the wall surface is increased, and radicals are not easily deposited on the wall surface. Therefore, the erosion amount of the wall is increased.
Japanese Patent Application Publication No. H8-22980, discloses that during a plasma process, a high frequency bias power is applied to a lower electrode and ions are attracted to the lower electrode. When cleaning is required due to deposits accumulated on an upper electrode or an inner wall of the processing chamber during the plasma process, a switch is switched such that the upper electrode is biased with a negative voltage, thereby applying the high frequency power to the upper electrode and attracting the ions toward the upper electrode. Due to the attraction of the ions, the deposits accumulated on the upper electrode can be removed.
Japanese Patent Application Publication No. H8-22980 suggests a technique for switching a supply destination of the high frequency bias power between the plasma process and the cleaning process. However, this technique cannot be used to solve the problem in which the wall is eroded or the film is formed on the wall during the plasma process.
To that end, it is considered to control the high frequency power such that the excessive erosion of the wall and the excessive deposition of the film on the wall can be prevented. This is because the potential difference between the wall of the processing chamber and the plasma (Vwall−Vplasma) generally depends on the high frequency power supplied to the electrode.
However, the high frequency power needs to be set to an optimal level in order to generate a plasma. Thus, the potential applied to the wall is not actively controlled, and is determined in accordance with the level of the high frequency power or the shape of the processing chamber.
When different processes are consecutively performed in a same processing chamber, the respective processes have different optimal high frequency power levels and, thus, it is extremely difficult to control the potential difference between the wall of the processing chamber and the plasma to be within a desired range under all the process conditions. Hence, the processing chamber is designed to have a structure that enables the potential difference between the wall and the plasma to be optimal for a representative high frequency power. However, the recent mainstream is batch etching of a multilayer structure in which a plurality of different etching processes is consecutively performed on a multilayer structure in the same processing chamber. Accordingly, the process requiring an extremely high power condition and that requiring an extremely low power condition need to be consecutively performed in the same processing chamber. This causes an excessive increase or decrease of the potential difference between the wall and the plasma (Vwall−Vplasma), which results in an erosion of the wall or the deposition of the film on the wall.